Causes

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Tricuspid atresia occurs during fetal growth when your baby's heart is developing. While some factors, such as heredity or Down syndrome, may increase your baby's risk of congenital heart defects, such as tricuspid atresia, the cause of congenital heart disease is unknown in most cases.

The normal-functioning heart

Your heart is divided into four chambers, two on the right and two on the left. In performing its basic job — pumping blood throughout the body — your heart uses its left and right sides for different tasks. The right side moves blood to the lungs. In your lungs, oxygen enriches the blood, which then circulates to your heart's left side. The left side of the heart pumps blood into a large vessel called the aorta, which circulates the oxygen-rich blood to the rest of your body. Valves control the flow of blood into and out of the chambers of your heart. These valves open to allow blood to move to the next chamber or to one of the arteries, and they close to keep blood from flowing backward.

When things go wrong

In tricuspid atresia, the right side of the heart can't properly pump blood to the lungs because the tricuspid valve, located between the upper right chamber (atrium) and the lower right chamber (ventricle), is missing. Instead, a solid sheet of tissue blocks the flow of blood from the right atrium to the right ventricle. As a result, the right ventricle is usually very small and underdeveloped (hypoplastic).

Blood instead flows from the upper right chamber to the upper left chamber through a hole in the wall between them (septum). This hole is either a heart defect (atrial septal defect) or an enlarged natural opening (foramen ovale) that is supposed to close soon after birth. If a baby with tricuspid atresia doesn't have an atrial septal defect, the baby may need a procedure to create this opening.

Once the blood flows from the right atrium to the left atrium, the heart's left side must pump blood both to the rest of the body and to the lungs. The blood reaches the lungs from the left side of the heart through another natural opening between the right and left sides (ductus arteriosus) that is supposed to close soon after birth. When necessary, this passageway is kept open using medication in babies with tricuspid atresia.

Some babies with tricuspid atresia have another heart defect — a hole between the lower two chambers (ventricular septal defect). In these cases, blood can flow through the hole and into the right ventricle, which pumps it to the lungs.

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